Gas turbine engines are used to power aircraft, watercraft, power generators, pumps, and the like. Gas turbine engines operate by compressing atmospheric air, burning fuel with the compressed air, and then removing work from hot high-pressure air produced by combustion of the fuel in the air. Rows of rotating blades and non-rotating vanes are used to compress the air and then to remove work from the high-pressure air produced by combustion. Each blade and vane has an airfoil that interacts with the gasses as they pass through the engine.
Multistage rotors of devices such as axial flow compressors and turbines include a number of coaxial disks that are interconnected by face splines and tie bolts. The face splines preferably are of a type known as Curvic® couplings which accurately and positively align the disks. In the manufacture of turbomachines of the multistage type, such as centrifugal compressors, axial flow compressors and turbines, there are advantages in employing rotors of the composite structure, wherein the wheels are arranged in stack formation between stub shafts, which are fixed to the ends of a thru-bolt, sometimes referred to as a tie-bolt.
The wheel members in centrifugal gas compressors take the form of impellers. In turbines and axial flow compressors, the wheels consist of bladed disks. The wheel assembly has an axially extending bore passage having clearance with the thru bolt. This form of rotor structure results in the thru-bolt being of substantial length. Such long bolts have inherent problems associated with bolt resonance, and rotor imbalance, caused mainly by deflection or radial displacement of the bolt. The situation is aggravated by the high speed at which the rotor structure operates.
Curvic couplings for use in the assembly of rotating element components were developed to meet the need for permanent coupling mechanisms requiring extreme accuracy, maximum load capacity, and relatively economical rates of production. The Curvic design provides an accurate, light, compact, and self-contained connection in which the Curvic teeth serve as centering and driving devices. The most widely used type of Curvic coupling used in gas turbine engines is the fixed Curvic coupling.
The fixed Curvic coupling can be described as a precision face spline with teeth that possess a high degree of accuracy of tooth spacing, fine surface finish, and precision axial location. The predominant application of fixed Curvic couplings is in the assembly of the elements that comprise a gas turbine engine rotating group, i.e., impellers, turbine wheels, and couplings. Many standard turboprop and turbofan engines are examples of this application, in which the disk-like members of the rotating group are mounted solely on fixed Curvic coupling teeth.